Leaf structure and why it matters

Most of the worlds biomass begins production in leaves

Leaves exchange gases via stomata


CO2 IN, O2 and H2O OUT

How stomata work…


- CO2 enters leaf airspaces and then chloroplasts through concentration gradient
- Stomata are not always open….WHY???

Leaf gas exchange impacts the global atmopshere

Vascular tissues in leaves are confined to veins


  • Transpiration (leaf water loss from evaporation) is a by‐product of opening stomatal pores

  • H2O transport needs to be sufficient to keep stomata open

  • Veins represents investment in H2O supply

Angiosperms have evolved higher vein densities


  • Helps explain angiosperm dominance
  • Transpiration alters global climate systems! About 10% of water in atmosphere comes from leaves

Leaf economic theory: Return on Investment

What do plants do when they are hungry?

Photosynthesis is the basis for most biomass on Earth




  • Greek = “building with light”
  • Autotroph = “self nourishing”
  • Photoautotroph = “self nourishing with light”


  • Photosynthesis is an energy pathway
    • 2 linked reactions
    • light-dependent reaction + Calvin cycle

Chloroplasts: Where the magic happens

The nature of light


When light hits chlorophyll a few things can happen

When pigments absorb photons an electron gets excited

Photosystems (2) convert light energy to chemical energy

Photosystem = packaging of chlorophyll pigments and proteins in thylakoid membranes

Light reactions: make ATP + NADPH for Calvin cycle


Calvin cycle occurs in the stroma

Calvin cycle = uses the chemical energy stored by the light-dependent reactions to form sugars

Calvin cycle is a 3-step process

  • 1. Fixation: CO2 reacts with 5C molecule (RUBP)
    • catalyzed by Rubisco enzyme
    • new 6C split into two acids (3C each)


  • 2. Reduction: ATP and NADPH convert 3C acids into 3C sugars
    • reduction of acids (requires electron carrier)
    • requires energy


  • 3. Regeneration: Some 3C sugars recycled to make RUBP (5C)
    • one 3C sugar leaves to make carbohydrates/sugars
    • recycling step also needs ATP

Making sugars requires several cycles

Three spins needed to remake RUBP and six spins to make glucose (a 6C molecule)

Rubisco - the enzyme that dominates carbon fixation






  • Most abundant enzyme in the world


  • Most CO2 converted into biomass is fixed by Rubisco


  • Large molecule : 16 polypeptides : 8 active sites

Rubisco: the clunky and slow carbon fixer





  • Only 3-10 reactions per second


  • 20% error rate
    • gets worse at high temperatures


  • Photorespiration - Rubisco cheats on CO2 with O2

C3 photosynthesis how did we get here…





  • Evolution doesn’t necessarily create the best solution


  • Oxygenation products are completely wasteful


  • 2x energy to produce the same amount of sugar than if Rubisco just reacted with CO2

Calvin Benson won a Nobel prize, his TA won best practical joke